Method for manufacturing wavelength conversion element

ABSTRACT

Provided is a method for manufacturing a wavelength conversion element  3  for converting a fundamental wave into a second harmonic wave, the method including the aging step (step  4 ) of irradiating a nonlinear optical crystal substrate  1  with a first light beam  4  having the same wavelength as the fundamental wave until the amount of variation per unit time in the phase matching temperature becomes a predetermined reference value or smaller while keeping the temperature of the nonlinear optical crystal substrate  1  at around the phase matching temperature after forming a periodical polarization-reversed structure in the nonlinear optical crystal substrate  1  (step  2 ).

TECHNICAL FIELD

The present invention relates to a method for manufacturing a secondharmonic wave generation wavelength conversion element (hereinafter,will be referred to as a SHG wavelength conversion element or awavelength conversion element) used for, for example, a laser lightsource device.

BACKGROUND ART

Gas laser light source devices such as argon gas laser and krypton gaslaser have been conventionally known. However, the devices have lowenergy conversion efficiency of 0.1% and require a cooling mechanism.Thus, the devices are difficult to be reduced in size. For this reason,wavelength conversion laser devices using nonlinear optical effectswhich are highly efficient as video or medical laser have attractedattention. A nonlinear optical crystal having birefringence is requiredto obtain the nonlinear optical effects. SHG wavelength conversionelements have been used in which a ferroelectric nonlinear crystal suchas a lithium niobate (LiNbO₃:PPLN) crystal is periodicallypolarization-reversed (e.g., see Patent Literature 1).

The SHG wavelength conversion element has a narrow wavelength phasematching temperature range of ±1° C. with respect to a fundamental wave,and thus requires temperature control using a temperature controlmechanism such as a Peltier element (e.g., see Patent Literature 2).

Output from wavelength conversion elements using polarization-reversedhighly nonlinear optical crystals such as LiNbO₃ or LiTaO₃ becomesunstable due to photorefractive damage. In particular, it is known thatrefractive index variation occurs in about several seconds to severalminutes after the incidence of a second harmonic wave such as greenlight.

Meanwhile, it is reported that metal additives such as magnesium,indium, scandium, and zinc are added to suppress the occurrence ofoptical damage. In particular, MgO-doped LN crystals have high nonlinearoptical constant and favorable crystallinity most promisingly. It isreported that the occurrence of optical damage can be suppressed in acongruent PPLN crystal containing at least 5.0 mol of a metal additive(e.g., see Patent Literatures 3 and 4, and Non-Patent Literature 1).

CITATION LIST Patent Literatures

Patent Literature 1: Japanese Patent Application Laid-Open PublicationNo. 2001-144354

Patent Literature 2: Japanese Patent Application Laid-Open PublicationNO. 8-171106

Patent Literature 3: Japanese Patent Application Laid-Open PublicationNo. 5-155694

Patent Literature 4: Japanese Patent Application Laid-Open PublicationNo. 7-89798

Non-Patent Literature

Non-Patent Literature 1: “Appl. Phys. Lett.” vol. 44, p. 847, 1984, D.A. Bryan, et al.

SUMMARY OF INVENTION Technical Problem

In the configuration of the related art, however, even though metaladditives are added, when the output of the second harmonic wave of thewavelength conversion element becomes 1 W or larger, the refractiveindex of the wavelength conversion element increases with time. Thus,the phase matching temperature varies and the output decreases. In otherwords, in the configuration of the related art, at least 1 W of laserlight outputted using the wavelength conversion element isdisadvantageously reduced with time.

An object of the present invention is to solve the problem and suppressa reduction over time in output even when high-power laser light isoutputted for a long period of time.

Solution to Problem

In order to attain the object, the method for manufacturing a wavelengthconversion element according to the present invention is a method formanufacturing a wavelength conversion element for converting afundamental wave to a second harmonic wave, the method comprising theaging step of irradiating a nonlinear optical crystal with a first lightbeam having the same wavelength as the fundamental wave until the amountof variation per unit time in the phase matching temperature becomes apredetermined value or smaller while keeping the temperature of thenonlinear optical crystal at around the phase matching temperature afterforming a periodical polarization-reversed structure in the nonlinearoptical crystal.

Preferably, the output of the second harmonic wave in the aging step isnot smaller than 0.5 W but smaller than 3 W.

Preferably, the integrated amount of output light of the second harmonicwave which is the product of the output of the second harmonic wave andaging time in the aging step is 600 W·hr or larger.

Preferably, the phase matching temperature is higher than 40° C. but nothigher than 80° C.

The method for manufacturing a wavelength conversion element accordingto the present invention is a method for manufacturing a wavelengthconversion element for converting a fundamental wave into a secondharmonic wave, the method comprising the aging step of irradiating anonlinear optical crystal with a first light beam having a wavelength inthe vicinity of the wavelength of the fundamental wave and a secondlight beam having a wavelength in the vicinity of the second harmonicwave until the amount of variation per unit time in the phase matchingtemperature becomes a predetermined reference value or smaller afterforming a periodical polarization-reversed structure in the nonlinearoptical crystal.

Furthermore, the first light beam and the second light beam may enter inparallel to each other from a propagation direction thereof.

Furthermore, the first light beam and the second light beam may enter soas to cross each other in the nonlinear optical crystal.

Preferably, the method for manufacturing a wavelength conversion elementfurther includes the heating step of retaining the nonlinear opticalcrystal at a predetermined heating temperature for predetermined heatingtime after forming the periodical polarization-reversed structure in thenonlinear optical crystal but before the aging step.

Preferably, the heating temperature is 85° C., and the heating time is125 hours or longer.

Preferably, the wavelength conversion element is stored at 80° C. orlower after the aging step.

Advantageous Effects of Invention

As described above, the wavelength conversion element is irradiated withthe first light beam having the same wavelength as the fundamental waveafter the formation of the periodical polarization-reversed structure inthe nonlinear optical crystal, so that the variation of the phasematching temperature can be saturated beforehand. Thus, it is possibleto suppress a reduction over time in output even when high-power laserlight is outputted for a long period of time.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart showing a method for manufacturing a wavelengthconversion element according to a first embodiment.

FIG. 2 is a cross-sectional view showing a process of the method formanufacturing a wavelength conversion element according to the firstembodiment.

FIG. 3 is a cross-sectional view illustrating aging according to thefirst embodiment.

FIG. 4 shows the amount of variation per unit time in phase matchingtemperature relative to the irradiation time of a first light beamaccording to the first embodiment.

FIG. 5 shows the relationship of the amount of variation from theinitial phase matching temperature relative to the integrated amount ofoutput light of a second harmonic wave according to the firstembodiment.

FIG. 6 shows the time variation of high-frequency output during acontinuous operation of the wavelength conversion element.

FIG. 7 shows the amount of variation in the phase matching temperaturerelative to the storage temperature of the wavelength conversionelement.

FIG. 8 is a cross-sectional view illustrating the aging step in a methodfor manufacturing a wavelength conversion element according to a secondembodiment.

FIG. 9 is a cross-sectional view illustrating the aging step in a methodfor manufacturing a wavelength conversion element according to a thirdembodiment.

FIG. 10 is a flowchart showing a method for manufacturing a wavelengthconversion element according to a fourth embodiment.

FIG. 11 is a cross-sectional view showing a wavelength conversion unitaccording to the fourth embodiment.

FIG. 12 is a flowchart showing a method for manufacturing a wavelengthconversion element according to a fifth embodiment.

FIG. 13 shows the relationship of the amount of variation in the phasematching temperature of the wavelength conversion element from theinitial stage relative to heating time in the heating step according tothe fifth embodiment.

FIG. 14 shows a difference in phase matching temperature variationbetween when heating is performed and when heating is not performed.

DESCRIPTION OF EMBODIMENTS

Background of the Invention

First, the background of the present invention will be described.

The inventors revealed by experiment that a reduction in output duringhigh-power wavelength conversion, which is the problem to be solved bythe present invention, was caused by a change in the phase matchingtemperature of a wavelength conversion element. The wavelengthconversion element used in the experiment was a Mg-doped LiNbO₃ crystalhaving a periodically polarization-reversed structure with a period ofabout seven microns and a phase matching temperature of about 50° C. Thephase matching temperature indicates a temperature at which theconversion efficiency from a fundamental wave to a second harmonic wavepeaks, and the temperature varies depending on the wavelength of thefundamental wave and the period of polarization reversal. In theexperiment, such a wavelength conversion element was used, and lightwith a fundamental wave of 7 W (having a wavelength of 1064 nm) wascollected in the wavelength conversion element, to perform wavelengthconversion for obtaining a second harmonic wave having a wavelength of532 nm (about 2 W). At this point, when time variation in output wasobserved, the output was reduced to not higher than half of the initialoutput in several hours. Concurrently, the phase matching temperature ofthe wavelength conversion element became higher than the settemperature. The change of the phase matching temperature is thought tohave been induced by refractive-index variation caused by the high-powerfundamental wave and the second harmonic wave. This is conceived for thefollowing reasons. First, it is reported that the refractive-indexvariation of radiated light is caused by optical damage. However,optical damage does not occur on light having a wavelength of 532 nm inMg-doped LiNbO₃. Further, the refractive-index variation due to opticaldamage is a reversible phenomenon in which the refractive index returnsto the original state when light radiation is stopped. In contrast, thevariation of the phase matching temperature observed in the experimentwas an irreversible phenomenon in which the refractive-index variationwas kept even when the wavelength conversion element had been left at50° C. for several months. Moreover, the refractive-index variation withtemperature observed in the experiment occurred not when light having awavelength of 532 nm or 1064 nm was singly radiated but when thefundamental wave and the second harmonic wave were concurrentlyradiated. It is considered from these factors that the reduction inoutput in the experiment, which had not been observed, was caused not byoptical damage but by the refractive-index variation due to theconcurrent radiation of the fundamental wave and the second harmonicwave. Furthermore, the phase matching temperature has been specific to awavelength conversion element, and it has not been known that the phasematching temperature varies when the output of the fundamental wave isincreased. Even though the phase matching temperature varied, wavelengthconversion at another phase matching temperature did not cause areduction in conversion efficiency. However, the variation of the phasematching temperature caused a difference between the set temperature andthe phase matching temperature, thereby having reduced the output. Ashas been discussed, it is found that when a high-power second harmonicwave is outputted, it is important to avoid the variation of the phasematching temperature. The present invention is characterized in that thephase matching temperature is prevented from varying in the case where ahigh-power second harmonic wave is outputted.

The following will specifically describe embodiments of a method formanufacturing a wavelength conversion element according to the presentinvention with reference to the accompanying drawings.

First Embodiment

First, a method for manufacturing a wavelength conversion elementaccording to a first embodiment of the present invention will bedescribed with reference to FIGS. 1 to 7.

FIG. 1 is a flowchart showing the method for manufacturing a wavelengthconversion element according to the first embodiment. FIGS. 2( a) to2(c) are process cross-sectional views showing the process of the methodfor manufacturing a wavelength conversion element according to the firstembodiment wherein FIG. 2( a) is a cross-sectional view of a nonlinearoptical crystal substrate to be the material of the wavelengthconversion element (step 1 in FIG. 1), FIG. 2( b) is a cross-sectionalview of the nonlinear optical crystal substrate after the step offorming a polarization-reversed portion (step 2 in FIG. 1), and FIG. 2(c) is a cross-sectional view of the nonlinear optical crystal substrateafter the aging step (step 3 in FIG. 1). FIG. 3 is a cross-sectionalview illustrating the aging step according to the first embodiment.

The respective steps of FIG. 1 in the method for manufacturing awavelength conversion element will be sequentially described.

(1) Step 1: Nonlinear Optical Crystal Substrate Preparation Step

First, a nonlinear optical crystal substrate to be the material of awavelength conversion element is prepared.

In the first embodiment, a wafer used for manufacturing a nonlinearoptical crystal substrate 1 is a LiNbO₃ crystal which has a thickness of1 mm and a diameter of 76.2 mm, contains 5.0 mol % of magnesium oxide,and has crystal orientation along the z axis.

FIG. 2( a) is the cross-sectional view of the nonlinear optical crystalsubstrate 1 used in the first embodiment. The nonlinear optical crystalsubstrate 1 is a rectangular parallelepiped with a thickness of about 1mm, a width of about 10 mm, and a length of about 25 mm, which isobtained by cutting out the wafer with a thickness of 1 mm and adiameter of 76.2 mm. FIGS. 2( a) to 2(c) are the cross-sectional viewsof the rectangular parallelepiped (1 mm in thickness×25 mm in length).

(2) Step 2: Polarization-Reversed Portion Formation Step

Next, polarization-reversed portions 2 are periodically formed insidethe nonlinear optical crystal substrate 1 (in other words, aperiodically polarization-reversed structure is formed).

In this step, first, an electrode pattern (not shown) is formed inportions of the nonlinear optical crystal substrate 1 where thepolarization-reversed portions 2 are formed. In the first embodiment,the period of the polarization-reversed portions 2 (corresponding to Ain FIG. 2( b)) is set to 7 μm in order to manufacture a wavelengthconversion element 3 used for a laser light source device which inputslight having a wavelength of 1064 nm as a fundamental wave to thewavelength conversion element 3 and outputs a second harmonic wavehaving a wavelength of 532 nm from the wavelength conversion element 3.

In the formation of the electrode pattern, a sputtering device is usedto form tantalum (Ta) thin films on surfaces 1 a of the nonlinearoptical crystal substrate 1, and a coater/developer is used to applyphotoresists over the tantalum thin films. Next, a mask with a repeatedpattern to be an electrode and the substrate with the photoresistsapplied thereon are made to contact each other and are exposed by anexposure unit. Thereafter, the photoresists with the pattern on the maskprinted thereon are developed by the coater/developer and are etched toform the electrode pattern.

A pulsed electric field is applied to the electrode pattern to form theperiodical polarization-reversed portions 2. Atom migration in thecrystal due to the application of the pulsed electric field reverses thepolarization orientation of the electrode pattern portion in the crystalorientation, so that the periodical polarization-reversed portions 2 areformed.

The electrode pattern is then removed. In the case where the electrodepattern is formed of tantalum, a fluoro-nitric acid solution is used.

As described above, the periodical polarization-reversed portions 2 areformed in the nonlinear optical crystal substrate 1 (in other words, theperiodical polarization-reversed structure is formed) in this step asshown in FIG. 2( b).

(3) Step 3: End Surface Treatment Step

Next, two ends 1 b of the nonlinear optical crystal substrate 1 areoptically polished, and then anti reflective films are formed on theoptically polished surfaces by the sputtering device.

This allows light such as a laser beam to be inputted to or outputtedfrom the nonlinear optical crystal substrate 1.

(4) Step 4: Aging Step

As shown in FIG. 3, a first light beam 4 having the same wavelength asthe fundamental wave is irradiated on the nonlinear optical crystalsubstrate 1 while the temperature of the nonlinear optical crystalsubstrate 1 is kept at around the phase matching temperature thereof.The phase matching temperature varies due to the irradiation of thefundamental wave, but the amount of variation is reduced as theirradiation time passes. Thus, as in step 5 which will be describedbelow, the fundamental wave continues to be irradiated until the amountof variation per unit time in the phase matching temperature of thenonlinear optical crystal substrate 1 becomes a predetermined referencevalue or smaller.

As described above, the fundamental wave is an optical wave which isinputted to the wavelength conversion element 3 by the laser lightsource device for which the nonlinear optical crystal substrate 1 (thatis, the wavelength conversion element 3 after the aging step) is used.In the first embodiment, as described above, the light having awavelength of 1064 nm as the fundamental wave is inputted to thewavelength conversion element 3, and the second harmonic wave having awavelength of 532 nm is outputted from the wavelength conversion element3. Thus, the wavelength of the first light beam 4 is 1064 nm.

As shown in FIG. 3, a light collection optical system 5 is placed on theside of the surface of the nonlinear optical crystal substrate 1, onwhich the first light beam 4 is incident, to collect the first lightbeam 4 in the nonlinear optical crystal substrate 1.

The nonlinear optical crystal substrate 1 is placed on a temperaturecontroller 6 such that the temperature of the nonlinear optical crystalsubstrate 1 is electronically variable. With this configuration, thetemperature of the nonlinear optical crystal substrate 1 is controlledto around the phase matching temperature by the temperature controller6.

As described above, the periodical polarization-reversed structureincluding the periodical polarization-reversed portions 2 is formed inthe nonlinear optical crystal substrate 1. The collected first lightbeam 4 is converted to a second harmonic wave 7 in the nonlinear opticalcrystal substrate 1.

Further, an area where the first light beam 4 passes through thenonlinear optical crystal substrate 1 is set as a first light beampropagation area 8, and an area where the second harmonic wave 7 passesthrough the nonlinear optical crystal substrate 1 is set as a secondharmonic beam propagation area 9.

(5) Step 5: Aging Step Continuation Determination Step

The above-described aging step is performed while the amount ofvariation in the phase matching temperature of the nonlinear opticalcrystal substrate 1 with respect to time is determined. Specifically,the aging step is performed until the amount of variation per unit timein the phase matching temperature of the nonlinear optical crystalsubstrate 1 becomes the reference value or smaller.

At an initial stage when the first light beam 4 starts entering, thetemperature of the nonlinear optical crystal substrate 1 is controlledwith a target temperature set at the phase matching temperature beforethe aging step continuation determination step. Thereafter, thetemperature of the nonlinear optical crystal substrate 1 is regularlyvaried by the temperature controller 6 (every ten hours in the firstembodiment) to measure output at measured temperatures, and thetemperature at which the output peaks is calculated as the phasematching temperature at that point. The calculated temperature isdetermined to be the phase matching temperature, the target temperatureis changed, and the first light beam 4 continues entering while thenonlinear optical crystal substrate 1 is kept at the changed targettemperature which is the phase matching temperature at that stage. Atthis point, a difference between the phase matching temperature theprevious time (ten hours before) and the phase matching temperature thistime is determined, and the time variation is calculated. When thevariation (that is, the amount of variation per unit time in the phasematching temperature) is larger than the predetermined reference value,the first light beam 4 continues entering. When the variation (that is,the amount of variation per unit time in the phase matching temperature)is not larger than the predetermined reference value, the first lightbeam 4 stops entering.

As described above, after the completion of this step, the wavelengthconversion element 3 (FIG. 2( c)) having an unvaried phase matchingtemperature can be manufactured.

In the first embodiment, the reference value of the amount of variationper unit time in the phase matching temperature of the nonlinear opticalcrystal substrate 1 is 0.0025° C./hr. The continuation of the aging stepis determined such that the aging step (that is, the incidence of thefirst light beam 4) continues until the amount of variation per unittime in the phase matching temperature of the nonlinear optical crystalsubstrate 1 becomes 0.0025° C./hr or smaller.

The following will describe the reason that the reference value of theamount of variation per unit time in the phase matching temperature ofthe nonlinear optical crystal substrate 1 is 0.0025° C./hr.

In the case where the amount of variation per unit time in the phasematching temperature of the nonlinear optical crystal substrate 1 islarger than 0.0025° C./hr, since the variation with time of the phasematching temperature of the nonlinear optical crystal substrate 1 isextremely large, the variation with time of the phase matchingtemperature of the nonlinear optical crystal substrate 1 cannot becomplemented by Auto Power Control (APC) which is generally used for thecontrol of light outputted from a laser light source. However, in thecase where the amount of variation per unit time in the phase matchingtemperature of the nonlinear optical crystal substrate 1 is not largerthan 0.0025° C./hr, the variation with time of the phase matchingtemperature can be complemented. Conversely, in the case where theoutput is not complemented according to the variation of the phasematching temperature by APC, the reference value may be reduced, and thewavelength conversion element 3 may be subjected to the aging step suchthat a reduction in output according to the variation of the phasematching temperature during an operation can be tolerable to the laserlight source device.

The above description is about the method for manufacturing a wavelengthconversion element according to the first embodiment of the presentinvention. The wavelength conversion element manufactured thus is thenmounted on a wavelength conversion unit and is used for the laser lightsource device.

Further, FIG. 4 shows the amount of variation per unit time in the phasematching temperature with respect to the irradiation time of the firstlight beam according to the first embodiment. The graph shows the amountof variation per unit time in the phase matching temperature of thenonlinear optical crystal substrate 1 with respect to the irradiationtime of the first light beam 4 in the case where the aging step isperformed such that the second harmonic wave 7 of the wavelengthconversion element 3 in the first embodiment becomes 1 W.

As shown in FIG. 4, the time variation of the phase matching temperaturegradually decreases with the irradiation time of the first light beam 4,and the time variation of the phase matching temperature hardly occursafter the elapse of about 600 hours. It is also founded out that sincethe amount of variation is always on the plus side, the phase matchingtemperature gradually shifts (varies with time) from the initial statetoward the high temperature side. This is because the variation withtime of the refractive index of the wavelength conversion element isobserved as the variation of the phase matching temperature. As shown inFIG. 4, the time variation of the phase matching temperature is asaturation phenomenon in which the variation of the phase matchingtemperature is saturated by the irradiation of the first light beam 4for a predetermined period of time, thereby significantly improving thetime variation of the phase matching temperature in practical use.

FIG. 5 shows the relationship of the amount of variation from theinitial phase matching temperature relative to the integrated amount ofoutput light of the second harmonic wave according to the firstembodiment. The relationship of the amount of variation from the initialphase matching temperature relative to the integrated amount of outputlight of the second harmonic wave is shown with the output of the secondharmonic wave 7 in the first embodiment set as parameters (0.5 W, 1 W,and 2 W). The integrated amount of output light of the second harmonicwave is the product (W·hr) of the output of the second harmonic wave (W)and the irradiation time of the first light beam 4 (hr). In FIG. 5, theabscissa indicates the integrated amount of output light of the secondharmonic wave, and the ordinate indicates the amount of variation fromthe initial phase matching temperature.

As shown in FIG. 5, the amount of variation from the initial phasematching temperature depends on the integrated amount of output light ofthe second harmonic wave. This makes it possible to reduce theirradiation time of the first light beam by the radiation of the firstlight beam such that the output of the second harmonic wave becomeshigh.

Moreover, as shown in FIG. 5, in the case where the wavelengthconversion element 3 of the first embodiment is used, when theintegrated amount of output light of the second harmonic wave is 600W·hr or larger, the variation of the phase matching temperature does notoccur (is saturated) and the amount of variation from the initial phasematching temperature is 1° C. Thus, the aging step is performedbeforehand such that the integrated amount of output light of the secondharmonic wave becomes 600 W·hr or larger, and consequently the variationof the phase matching temperature is saturated and the phase matchingtemperature further increases by 1° C. Hence, the phase matchingtemperature is increased by 1° C. from the initial state during apractical operation, so that high-power laser light can be outputted fora long period of time while a reduction in output is suppressed.

Auto Power Control (APC) has been conventionally used to suppress areduction in output light. The common APC can complement a reduction inthe output of the second harmonic wave substantially equivalent to 0.4°C. which is the amount of variation in the phase matching temperature.Thus, the above-described aging step and the APC can be combined.

Specifically, as shown in FIG. 5, the amount of variation from theinitial phase matching temperature is 1° C. in the case where theintegrated amount of output light of the second harmonic wave is 600W·hr or larger, and the amount of variation from the initial phasematching temperature is 0.6° C. in the case where the integrated amountof output light of the second harmonic wave is 200 W·hr. Thus, after thefirst light beam 4 is radiated when the integrated amount of outputlight of the second harmonic wave is 200 W·hr, the amount of variationwith time in the phase matching temperature is 0.4° C. For this reason,the aging step can be beforehand performed with input light having thesame wavelength as the fundamental wave of the first light beam 4 suchthat the integrated amount of output light of the second harmonic waveis 200 W·hr, and then APC can be performed during a practical operation.Such a control causes only a reduction in the output of the secondharmonic wave equivalent to 0.4° C. which is the amount of variation inthe phase matching temperature of the wavelength conversion elementafter the aging step. Thus, the reduction in the output can becomplemented by APC to maintain high output for a long period of time.In other words, when the first light beam 4 is radiated such that theintegrated amount of output light of the second harmonic wave is 200W·hr or larger, the reduction over time in the output of the secondharmonic wave can be suppressed to provide a sufficiently practicalwavelength conversion element 3.

It is possible to carry out the above-described method for manufacturinga wavelength conversion element with other condition settings. Thefollowing will describe the details of the other conditions.

In the case where the first light beam was radiated such that the outputof the second harmonic wave was below 0.5 W, a reduction over time inthe output of the second harmonic wave was not suppressed. Further, astable reduction over time in the output of the second harmonic wavecould not be suppressed in the case where the output of the secondharmonic wave was 3 W or larger. Thus, when radiating the first lightbeam 4, the output of the second harmonic wave has to be not smallerthan 0.5 W but smaller than 3 W.

FIG. 6 shows the time variation of high frequency output during acontinuous operation of the wavelength conversion element. A wavelengthconversion element according to the related art is compared with thewavelength conversion element 3 of the first embodiment. The abscissaindicates continuous operation time, and the ordinate indicates highfrequency output. The wavelength conversion element 3 of the firstembodiment subjected to the aging step for 600 hours was used, in astate in which the first light beam 4 was adjusted such that the outputof the second harmonic wave 7 was 1 W. The output of the second harmonicwave of the initial wavelength conversion element is 1.5 W.

As is clear from FIG. 6, the output of the wavelength conversion elementaccording to the related art was 1.35 W after the elapse of 100 hours, a10% reduction from the initial output. In contrast, a reduction in theoutput of the wavelength conversion element 3 according to the firstembodiment could not be observed even after the elapse of 1000 hours.Thus, a reduction over time in the output of the second harmonic wave 7could not be observed in the wavelength conversion element 3 subjectedto the aging step according to the present invention even when thewavelength conversion element was operated for a long period of time.Evaluations were performed on the wavelength conversion element 3subjected to the aging step for 200 hours in the state in which thefirst light beam 4 was adjusted such that the output of the secondharmonic wave 7 was 1 W, that is, in a state in which the amount ofvariation in the phase matching temperature of the wavelength conversionelement 3 was below 0.0025° C./hr which is the reference value of theamount of variation per unit time in the phase matching temperature.Similarly to the above wavelength conversion element subjected to theaging step for 600 hours, a reduction over time in the second harmonicwave output was not observed in the case of the above-describedcomplementation with APC, even when the wavelength conversion elementwas operated for a long period of time.

As described above, the first light beam 4 having the same wavelength asthe fundamental wave is radiated on the wavelength conversion element 3after the periodical polarization-reversed structure is formed on thenonlinear optical crystal, so that the variation of the phase matchingtemperature can be saturated beforehand. Thus, a reduction over time inoutput can be suppressed even when high-power laser light is outputtedfor a long period of time.

Moreover, the period of the polarization-reversed portions 2 of thenonlinear optical crystal substrate 1 was changed to change the phasematching temperature of the wavelength conversion element 3, and effectson the phase matching temperature due to the radiation of the firstlight beam 4 were examined. Consequently, even though the aging step wasperformed such that the integrated amount was 1000 W·hr or larger, theamount of variation in the phase matching temperature was not saturatedwhen the phase matching temperature was 40° C. or lower. Further, whenthe phase matching temperature exceeded 80° C., the effects caused bythe radiation of the first light beam 4 could not be stably produced.According to the results, the period of the polarization-reversedportions 2 of the nonlinear optical crystal substrate 1 has to bedesigned such that the phase matching temperature is higher than 40° C.but not higher than 80° C.

Evaluations were performed on the storage temperature of the wavelengthconversion element 3 subjected to the aging step. The wavelengthconversion element 3 irradiated with the first light beam 4 such thatthe integrated amount was 600 W·hr was stored in high-temperatureenvironment, and then the amount of variation in the phase matchingtemperature was evaluated. The phase matching temperature of thewavelength conversion element 3 shifted to the high temperature side byabout 1° C. from the initial phase matching temperature with theirradiation of the first light beam 4.

FIG. 7 shows the amount of variation in the phase matching temperaturewith respect to the storage temperature of the wavelength conversionelement. The abscissa indicates the storage temperature, and theordinate indicates the amount of variation in the phase matchingtemperature. The temperature profile of high-temperature storage wasthat the storage temperature was changed from a room temperature of 25°C. to a target temperature in two minutes, and was returned to the roomtemperature of 25° C. in two minutes after having being kept for 60minutes.

As shown in FIG. 7, the phase matching temperature did not vary beforethe storage temperature reached 80° C., as compared with the phasematching temperature after the radiation of the first light beam 4. Inthe case where the storage temperature was 90° C. or higher, the amountof variation in the phase matching temperature before the aging step wascompletely recovered.

Thereafter, when the wavelength conversion element restored to theinitial phase matching temperature was continuously operated again, thephase matching temperature shifted from the initial phase matchingtemperature to the high temperature side again. Thus, when thewavelength conversion element is restored to the initial phase matchingtemperature in the high-temperature environment after the aging step,the effects of the aging step are lost, thereby causing the variation ofthe phase matching temperature again. According to the result, thewavelength conversion element 3 has to be stored at a temperature of 80°C. or lower after the aging step.

In the first embodiment, the element is composed of LiNbO₃ having acongruent composition with a magnesium oxide content of 5.0 mol-%.However, the variation of the phase matching temperature can besaturated by the aging step under certain conditions, even when theelement is composed of LiTaO₃ having a congruent composition with amagnesium oxide content of 5.0 mol %, or LiNbO₃, LiTaO₃, or KTiOPO₄having a stoichiometric composition with a magnesium oxide content of atleast 1 mol.

In the first embodiment, the wavelength conversion using the nonlinearoptical effect of the optical element is explained by way of example.However, an optical element having a polarization-reversed structure formatching the phases of light using the period of the polarizationreversal or matching the velocities of light and a microwave may beapplied. Further, in the first embodiment, the conversion (generation ofthe second harmonic wave) from infrared light (1064 nm) into visiblelight (532 nm) is explained by way of example. However, a system formatching the phases of light with sum frequency generation or differencefrequency generation using the period of the polarization reversal orparametric oscillation may be applied.

In the first embodiment, the wavelength of the first light beam 4 is1064 nm but may be 900 nm to 1200 nm in the vicinity of 1064 nm.

Second Embodiment

The following will describe a method for manufacturing a wavelengthconversion element according to a second embodiment of the presentinvention.

FIG. 8 is a cross-sectional view illustrating the aging step in themethod for manufacturing a wavelength conversion element 3 according tothe second embodiment.

The second embodiment is different from the first embodiment in that, inthe aging step of the method for manufacturing the wavelength conversionelement 3, a first light beam 4 having the same wavelength as afundamental wave and a second light beam 10 having the same wavelengthas a second harmonic wave are radiated on a nonlinear optical crystalsubstrate 1, so as to enter parallel to a direction in which the firstlight beam 4 and the second light beam 10 propagate, and the radiationcontinues until the amount of variation per unit time in the phasingmatching temperature of the nonlinear optical crystal substrate 1becomes a predetermined reference value or smaller. The steps explainedin the first embodiment can be performed other than the method of lightradiation in the aging step, and an explanation thereof is omitted.

In the second embodiment, for example, a light beam having a wavelengthof 1064 nm can be used as the first light beam 4, and a light beamhaving a wavelength of 532 nm can be used as the second light beam 10.

The first light beam 4 and the second light beam 10 are radiated, sothat the inside of the nonlinear optical crystal substrate 1 comescloser to a state in which the temperature is regulated and the secondharmonic wave (532 nm) is being generated from the light having thewavelength of 1064 nm. Thus, the same state as in the aging step of thefirst embodiment can be obtained and the phase matching temperature canbe saturated beforehand without keeping the temperature of the nonlinearoptical crystal substrate 1 at around the phase matching temperatureduring the aging step, so that high output can be maintained inwavelength conversion. Hence, a temperature control system is notnecessary for the nonlinear optical crystal substrate 1. As a result,the manufacturing cost for the aging step of the wavelength conversionelement 3 can be reduced and the wavelength conversion element 3 can beeasily manufactured.

In the second embodiment, the light having the same wavelength of 1064nm as the fundamental wave is used as the first light beam 4 but may belight having a wavelength (900 nm to 1200 nm) in the vicinity of thewavelength of the fundamental wave.

In the second embodiment, the light having the wavelength of 532 nm isused as the second light beam 10 but may be light having a wavelength(350 nm to 600 nm) in the vicinity of the wavelength of the secondharmonic wave.

Third Embodiment

The following will describe a method for manufacturing a wavelengthconversion element according to a third embodiment of the presentinvention.

FIG. 9 is a cross-sectional view illustrating the aging step in themethod for manufacturing a wavelength conversion element 3 according tothe third embodiment.

The third embodiment is different from the second embodiment in that, inthe aging step of the method for manufacturing the wavelength conversionelement 3, a first light beam 4 having the same wavelength as afundamental wave and a second light beam 10 having the same wavelengthas a second harmonic wave are radiated on a nonlinear optical crystalsubstrate 1, so as to cross each other in the nonlinear optical crystalsubstrate 1, and the radiation continues until the amount of variationper unit time in the phase matching temperature of the nonlinear opticalcrystal substrate 1 becomes a predetermined reference value or smaller.In the third embodiment, the wavelength of the first light beam 4 is1064 nm and the wavelength of the second light beam 10 is 532 nm.

This configuration eliminates the need to coaxially arrange the opticalaxes of the first light beam 4 and the second light beam 10 during thelight incidence on the nonlinear optical crystal substrate 1. Further,the phase matching temperature can be saturated beforehand withoutkeeping the temperature of the nonlinear optical crystal substrate 1 ataround the phase matching temperature during the aging step, so thathigh output can be maintained in wavelength conversion, similarly to thesecond embodiment. Hence, the optical system of the first light beam 4and the optical system of the second light beam 10 can be relativelyeasily designed, so that the manufacturing cost of the wavelengthconversion element 3 can be reduced further than that in the secondembodiment.

In the third embodiment, the first light beam 4 has the same wavelengthof 1064 nm as the fundamental wave but may have a wavelength (900 nm to1200 nm) in the vicinity of the wavelength of the fundamental wave.

In the third embodiment, the second light beam 10 has the wavelength of532 nm but may have a wavelength (350 nm to 600 nm) in the vicinity ofthe second harmonic wave.

Fourth Embodiment

The following will describe a method for manufacturing a wavelengthconversion element according to a fourth embodiment of the presentinvention.

FIG. 10 is a flowchart showing the method for manufacturing a wavelengthconversion element according to the fourth embodiment.

The fourth embodiment is different from the first embodiment in that, inthe method for manufacturing a wavelength conversion element 3, atemperature controller mounting step (step A in FIG. 10) is providedafter the formation of a periodical polarization-reversed structure on anonlinear optical crystal but before the aging step. Further, the fourthembodiment is characterized in that the aging step can be performed witha nonlinear optical crystal substrate 1 incorporated into a wavelengthconversion unit used for, for example, a laser light source device. Thefollowing will describe the temperature controller mounting step. Othersteps are the same as the steps and conditions in the first embodiment,and an explanation thereof is omitted. Moreover, the nonlinear opticalcrystal substrate can be irradiated with a second light beam 10 as inthe second and third embodiments.

In the temperature controller mounting step (step A), the nonlinearoptical crystal substrate 1 having a periodical polarization-reversedstructure formed on a nonlinear optical crystal is mounted on atemperature controller 12. The aging step is performed with thenonlinear optical crystal substrate 1 put on the temperature controller6 to evaluate the element characteristics in FIG. 3 of the first tothird embodiments. After the aging step, the wavelength conversionelement 3 fixed to the separately provided wavelength conversion unit isused for, for example, the laser light source device which is a finalproduct. In contrast, FIG. 11 of the fourth embodiment is different fromFIG. 3 in that the nonlinear optical crystal substrate 1 is bonded andfixed to a copper plate 13 of the temperature controller 12 and ismounted as the wavelength conversion unit, and then the aging step isperformed.

FIG. 11 is a cross-sectional view showing the wavelength conversion unitaccording to the fourth embodiment.

As shown in FIG. 11, a wavelength conversion unit 11 includes the copperplate 13 bonded onto the temperature controller 12 with an adhesive, andthe nonlinear optical crystal substrate 1 having the periodicalpolarization-reversed structure bonded onto the copper plate 13 with anadhesive.

Such a manufacturing method enables the temperature controller 12 of thewave conversion unit 11 to control the temperature of the nonlinearoptical crystal substrate 1 in the aging step 4, as compared to thefirst embodiment. Thus, it is possible to eliminate the step ofincorporating the nonlinear optical crystal substrate 1 into thewavelength conversion unit 11 at the stage of manufacturing a finalproduct. As a result, the wavelength conversion unit 11 can be easilymanufactured.

Fifth Embodiment

The following will describe a method for manufacturing a wavelengthconversion element according to a fifth embodiment of the presentinvention.

FIG. 12 is a flowchart showing the method for manufacturing a wavelengthconversion element according to the fifth embodiment.

The fifth embodiment is different from the first embodiment in that, inthe method for manufacturing a wavelength conversion element 3, aheating step (step B) is provided after the formation of a periodicalpolarization-reversed structure on a nonlinear optical crystal butbefore the aging step. The following will describe the heating step.Other steps are the same as the steps and conditions in the firstembodiment, and an explanation thereof is omitted. Further, a nonlinearoptical crystal substrate can be irradiated with a second light beam 10as in the second and third embodiments, and can be mounted on atemperature controller as in the fourth embodiment.

In the heating step (step B), a nonlinear optical crystal substrate 1having a periodical polarization-reversed structure formed on anonlinear optical crystal is placed on a temperature controller 6 asshown in FIG. 3, and heat is applied to the nonlinear optical crystalsubstrate under the conditions described below.

Effects of the fifth embodiment will be described with reference toFIGS. 13 and 14.

FIG. 13 shows the relationship of the amount of variation in the phasematching temperature of the wavelength conversion element from theinitial stage relative to heating time in the heating step according tothe fifth element. In FIG. 13, heating temperatures in the heating stepof 60° C., 70° C., 85° C., 90° C., and 100° C. are parameters.

As shown in FIG. 13, in the case where the heating temperature is 60°C., 70° C., 85° C., or 100° C., the phase matching temperature shifts tothe high temperature side. Further, in the case where the heatingtemperature in the heating step is 85° C., the phase matchingtemperature is saturated in about 125 hours (the variation of the phasematching temperature becomes constant). Further, in the case where theheating temperature in the heating step is 60° C. or 70° C., the heatingtime becomes longer but the phase matching temperature comes close tothe same saturation temperature. Thus, the heating step for at least 125hours is required. However, in the case where the heating temperature inthe heating step is 90° C., the phase matching temperature shifts to thelow temperature side, and then specifically returns to the initialstate. Furthermore, in the case where the heating temperature in theheating step is 100° C., the phase matching temperature shifts to thehigh temperature side in 20 hours, but when the heating time isextended, the phase matching temperature conversely shifts to the lowtemperature side, exhibiting a specific behavior. As described above, inthe case where the heating temperature is 90° C. or higher, the amountof variation in the phase matching temperature is not stable and stablevariation in the phase matching temperature cannot be obtained.

The following will describe a comparison between when heating isperformed and when heating is not performed (the first embodiment) inthe amount of variation per unit time in the phase matching temperatureof the nonlinear optical crystal substrate 1 relative to the irradiationtime of a first light beam 4.

FIG. 14 shows a difference in phase matching temperature variationbetween when heating is performed and when heating is not performed, andthe amount of variation per unit time in the phase matching temperatureof the nonlinear optical crystal substrate 1 with respect to theirradiation time of the first light beam 4 according to the fifthembodiment. In the fifth embodiment, the heating temperature in theheating step was 85° C., the heating time was 150 hours, and the agingstep was performed with a first light beam 4 having such an amount oflight that a second harmonic wave 7 became 1 W. As shown in FIG. 14,when heating was not performed, as compared to when heating wasperformed, the time variation was smaller and the time until thesaturation of the phase matching temperature was longer. Thus, in themethod for manufacturing the wavelength conversion element 3, thepredetermined heating step is provided after the formation of aperiodical polarization-reversed structure on the nonlinear opticalcrystal but before the aging step, so that the time of the aging stepcan be shortened. It is noted from the above-described experimentresults that when heating is performed at a heating temperature of 60°C. to 85° C. for heating time of 125 hours or longer, the aging time canbe shortened. It is also noted that the heating temperature ispreferably 85° C.

The results shown in FIGS. 13 and 14 will be discussed.

Generally, a periodical polarization-reversed structure is formed by anexternal electric field, so that areas having a spontaneous polarizationreversed with a micron-order short-period structure are adjacent to eachother to form LiNbO₃ and LiTaO₃ crystals. The boundary between the areashaving the reversed spontaneous polarization is called a domain wall.Further, the spontaneous polarization of the crystal is reversed, sothat the crystal has a distortion therein. The distortion includescharge localization caused by the movement of lithium ions and astructural distortion occurring on the domain wall due to a change ofthe crystal structure. The charge localization forms charge distributionin the direction of the spontaneous polarization and generates anelectric field facing the spontaneous polarization. The electric fieldreduces the refractive index of the crystal due to electro-opticaleffects. The charge localization is trapped in a shallow impurity leveland is gradually discharged with time, so that electric localization isreduced. This is considered to be a cause for the variation with time inwhich the phase matching temperature of the wavelength conversionelement gradually increases over a long period of time. The movement ofcharge trapped in the impurity level is effectively accelerated byincreasing the temperature to accelerate a reduction in the chargelocalization. This is the reason that the heating step of the presentinvention is effective. Heating is performed at 85° C. or lower, so thatthe reduction in the charge localization caused by the polarizationreversal or the heating step can be accelerated and the variation withtime of the phase matching temperature can be suppressed. In contrast,the heating temperature was increased to higher than 90° C., so that therefractive index of the crystal was reduced again and the variation withtime was reset to the original state (a state before the variation withtime). This is because free charge due to the crystal defects is rapidlyincreased when the temperatures of the LiNbO₃ and LiTaO₃ crystals areincreased to 90° C. or higher. The temperature increase to 90° C. orhigher is known as a cause for the reduction of optical damage. Theincreased free charge constitutes the state of charge localization inthe crystal again with the internal electric field of the spontaneouspolarization. Thus, the variation with time is considered to be reset tothe start condition.

As described above, in the method for manufacturing a wavelengthconversion element, the periodical polarization-reversed structure isformed in the nonlinear optical crystal and the heating step is providedbefore the aging step, so that the reduction of the charge localizationcaused by the polarization reversal or the heating step can beaccelerated. Thus, time for the aging step can be shortened.

In the fifth embodiment, heating is performed by the temperaturecontroller 6 but may be performed by, for example, a thermostatic bath.

INDUSTRIAL APPLICABILITY

The present invention is useful for, for example, a method formanufacturing a second harmonic wave generation wavelength conversionelement which can suppress a reduction over time in output and output astable second harmonic wave in the long term and is used for, forexample, a laser light source device.

1. A method for manufacturing a wavelength conversion element forconverting a fundamental wave to a second harmonic wave, the methodcomprising an aging step of irradiating a nonlinear optical crystal witha first light beam having the same wavelength as the fundamental waveuntil an amount of variation per unit time in a phase matchingtemperature becomes a predetermined value or smaller while keeping atemperature of the nonlinear optical crystal at around the phasematching temperature after forming a periodical polarization-reversedstructure in the nonlinear optical crystal.
 2. The method formanufacturing a wavelength conversion element according to claim 1,wherein output of the second harmonic wave in the aging step is notsmaller than 0.5 W but smaller than 3 W.
 3. The method for manufacturinga wavelength conversion element according to claim 1, wherein anintegrated amount of output light of the second harmonic wave which is aproduct of the output of the second harmonic wave and aging time in theaging step is 600 W hr or larger.
 4. The method for manufacturing awavelength conversion element according to claim 1, wherein the phasematching temperature is higher than 40° C. but not higher than 80° C. 5.A method for manufacturing a wavelength conversion element forconverting a fundamental wave into a second harmonic wave, the methodcomprising an aging step of irradiating a nonlinear optical crystal witha first light beam having a wavelength in the vicinity of a wavelengthof the fundamental wave and a second light beam having a wavelength inthe vicinity of the second harmonic wave until an amount of variationper unit time in a phase matching temperature becomes a predeterminedreference value or smaller after forming a periodicalpolarization-reversed structure in the nonlinear optical crystal.
 6. Themethod for manufacturing a wavelength conversion element according toclaim 5, wherein the first light beam and the second light beam enter inparallel to each other from a propagation direction thereof.
 7. Themethod for manufacturing a wavelength conversion element according toclaim 5, wherein the first light beam and the second light beam enter soas to cross each other in the nonlinear optical crystal.
 8. The methodfor manufacturing a wavelength conversion element according to claim 1,further comprising a heating step of retaining the nonlinear opticalcrystal at a predetermined heating temperature for predetermined heatingtime after forming the periodical polarization-reversed structure in thenonlinear optical crystal but before the aging step.
 9. The method formanufacturing a wavelength conversion element according to claim 8,wherein the heating temperature is 85° C., and the heating time is 125hours or longer.
 10. The method for manufacturing a wavelengthconversion element according to claim 1, wherein the wavelengthconversion element is stored at 85° C. or lower after the aging step.